The present invention relates to novel citrullinated peptides recognized by rheumatoid arthritis-specific autoantibodies.
Rheumatoid arthritis (hereinafter abbreviated to “RA”) is the most common of the chronic inflammatory forms of rheumatism. It is an autoimmune disease and the serum from affected patients contains autoantibodies, some of which are specific and may constitute a marker for this disease, allowing it to be diagnosed even at very early stages. Research studies have therefore been carried out for the purpose of identifying antigens recognized by these antibodies, in order to obtain purified preparations of the latter that can be used in conventional techniques of immunological diagnosis.
It has been shown that RA-specific autoantibodies recognize various isoelectric variants of (pro)filaggrin (for review, cf., for example, SERRE and VINCENT, In: Autoantibodies, PETER and SHOENFELD Eds, Elsevier Science Publishers, 271-276, 1996). These autoantibodies have for this reason been called: “anti-filaggrin autoantibodies (AFAs)”. Application EP 0 511 116 describes the purification and characterization of antigens of the filaggrin family, recognized by these antibodies, and their use for the diagnosis of rheumatoid arthritis.
Subsequently, the filaggrin epitopes identified by AFAs were identified as regions of the filaggrin molecule carrying citrullyl residues, resulting from the conversion of arginyl residues by a peptidylarginine deiminase (Girbal-Neuhauser E et al., J. Immunol, 162, 585-94, 1999; Schellekens G A et al., J Clin Invest, 101, 273-81, 1998). The analysis of various synthetic peptides derived from the sequence of human filaggrin has shown that deimination (citrullination) is necessary in order to form the epitopes recognized by AFAs, which are today also called anti-citrullinated protein autoantibodies (ACPAs). It is this name that will be used in the disclosure which follows.
It has also been observed that the environment of the citrullyl residues also plays an important role (Girbal-Neuhauser E, 1999, mentioned above; Schellekens Ga., 1998, mentioned above; Union A et al., Arthritis Rheum, 46, 1185-95, 2002); some amino acids that are “permissive” with respect to binding of the antibody, and the nature of which probably modifies the antibody binding affinity, have been identified. Among these amino acids, -gly-, -ser-, -his-, -thr- and -gln-residues are most commonly found in the immediate environment of the citrullyl residues of citrullinated “filaggrin” peptides which, to date, have been identified as carrying epitopes recognized by ACPAs (Sebbag M et al., Rev Rhum, 68, 106, 2001).
A large number of citrullinated peptides specifically recognized by ACPAs, and that can be used for the diagnosis of RA, have been obtained from filaggrin. However, it has also been observed that, although strictly specific RA, the reactivity of these various citrullinated peptides with respect to ACPAs is heterogeneous, different peptides being recognized by sera derived from different individuals. This implies that, in order to obtain a diagnostic reagent capable of identifying the presence of ACPAs in a large population, it is necessary to combine several different peptides.
In parallel, it has been shown that ACPAs are secreted by synovial tissue plasma cells (Masson-Bessière C et al., Clin Exp Immunol, 119, 544-52, 2000) and are specifically directed against citrullinated forms of fibrin α- and β-chains present in this tissue (Masson-Bessière C et al., J Immunol, 166, 4177-84, 2001).
The inventors have undertaken, with the aim of more thoroughly characterizing the epitopes recognized by ACPAs, to identify those presented by fibrin α- and β-chains. For this purpose, they have evaluated the reactivity of sera containing ACPAs with respect to citrullinated synthetic peptides derived from the sequence of the α-chain and from the sequence of the β-chain of fibrin. Given the known heterogeneity of the reaction profiles of citrullinated peptides derived from filaggrin, with ACPAs, the inventors used mixtures of sera selected so as to contain ACPAs representing the various reactivity profiles observed in the case of these filaggrin-derived peptides, in order to detect all the fibrin peptides that may be recognized by ACPAs.
The peptides tested were obtained from the sequence of the α-chain and from the sequence of the β-chain of fibrin. In total, 72 peptides, including 41 derived from the fibrin α-chain and 31 derived from its β-chain, were selected. Among the 72 citrullinated peptides analyzed, the inventors identified 13 peptides derived from the sequence of the α-chain and 5 peptides derived from that of the β-chain of fibrin as reacting significantly with one and/or the other of the mixtures of sera tested, and therefore as carrying epitopes recognized by some of the ACPAs present in this of these mixture(s).
The inventors subsequently individually analyzed each of the 18 reactive peptides identified with each of the sera constituting the mixtures tested. This analysis confirmed, for most of the peptides tested, the large interindividual variability in ACPA specificity previously observed in the case of filaggrin-derived citrullinated peptides.
Furthermore, it allowed the inventors to identify peptides which, unexpectedly, have a spectrum of reactivity with ACPAs that is much broader than those reported up until now in the case of filaggrin-derived citrullinated peptides. Indeed, the inventors have identified 5 citrullinated peptides (4 peptides derived from the fibrin α-chain and 1 derived from the fibrin β-chain) which are each individually recognized by at least 40% of the sera analyzed, and therefore appear to carry major epitopes. Among these peptides, two are recognized by the majority of the sera, and also exhibit complementary reactivity profiles encompassing all the sera analyzed. Each of these sera in fact recognizes one and/or the other of these peptides. This suggests that these two peptides carry structural motifs representative of a very large majority of the various motifs recognized by ACPAs.